A structure of a carbon nanotube (hereinafter, referred to as CNT) was first found in 1991. Synthesis, physical property, and application of the carbon nanotube have been actively studied. Also, it has been confirmed that the CNT is produced by adding transition metals such as ferrum (Fe), nickel (Ni) and cobalt (Co), at the time of discharging electricity. A full study started from a preparation of a significant amount of samples by a laser evaporation method in 1996. The CNT takes a form of a round wound hollow tube whose graphite surface is a diameter of a nano size. At this time, the CNT has electrical characteristics such as conductor property, semiconductor property, etc., according to the wound angle or structure of the graphite surface. Also, the CNT is divided into a single-walled carbon nanotube (SWCNT), a double-walled carbon nantube (DWCNT), a thin multi-walled carbon nanotube, a multi-walled carbon nanotube (MWCNT), and a roped carbon nanotube according to the number of the graphite walls.
In particular, the CNT has excellent mechanical strength or elastic strength, chemical stability, eco-friendliness, and electrical conductor and semiconductor property as well as has an aspect ratio larger than the existing any materials, wherein the aspect ratio reaches about 1,000 as a diameter of 1 mm to several tens nm and a length of several μm to several tens μm. Also, the CNT has a very large specific-surface area. As a result, the CNT is being interested as advanced new materials, which will lead the twenty-first century, in the field of next-generation information electronic materials, high-efficiency energy materials, high-functional complex materials, eco-friendly materials, and the like.
However, in spite of various advantages owned by the CNT, since the CNT has very large agglomeration phenomenon and very large hydrophobic property, the CNT is very poor in terms of the mixed property with other media as well as does not have solubility to organic solvents in addition to water. Therefore, in order to expand the applications of the CNT while having the advantages of the CNT, a method capable of increasing compatibility with various media and making dispersion efficiency good is needed. As a technology of increasing the compatibility with CNT, there is a functional group substituting technology capable of providing separate characteristics on a surface, for example, there are a method of increasing the specific-surface area of CNT using strong bases such as potassium hydroxide, sodium hydroxide, etc., under vacuum and inert gas atmosphere as described in KR Patent No. 450,029 and a method of treating a surface of CNT using strong acids or strong bases as described in KR Patent Publication Nos. 2001-102598, 2005-9711, and 2007-114553.
However, since the above technologies use strong acids, such as nitric acid, sulfuric acid, etc., or strong bases, such as potassium hydroxide, sodium hydroxide, etc., they are harmful to environment, are not easy to handle, and can cause the corrosion of a reactor. Further, they need further processes, such as a process of washing used acids and bases, or cause a large amount of harmful wastes. In addition, since they have long reaction time and limited throughput, the efficiency is low and in order to provide the functional group in addition to oxygen on the surface, since they need separate processes, much cost and time are consumed.